The radio frequency (RF) spectrum is the foundation for many wireless communications systems in use today, including radar and cellular communications systems. Specified frequency ranges, sometimes identified as bands or channels, in the RF spectrum may be allocated for use by different entities, for different purposes, or in different geographic locations. As used in this disclosure, “spectrum” refers to any frequencies, frequency bands, and frequency channels in the RF spectrum that may be used or allocated for wireless communications.
Because the available RF spectrum is finite, frequency allocations in the spectrum are highly valued and often highly regulated. In the United States, for example, the Federal Communications Commission (FCC) and the National Telecommunication and Information Administration (NTIA) regulate and manage spectrum allocations, allotments, and assignments. Frequency allocation is the process by which the entire RF spectrum is divided into frequency bands established for particular types of service. These frequency allocations are then further subdivided into channels designated for a particular service or “allotment.” Assignment refers to the final subdivision of the spectrum in which a party gets one or more frequency assignments, in the form of a license, to operate a radio transmitter on specific frequencies within a particular geographic location.
The system of spectrum allocation, allotment, and assignment is failing to keep pace with the increasing demand for spectrum. There is therefore a need to improve how the available spectrum can be efficiently allocated, allotted, and assigned in the face of growing demand. Unless otherwise noted, “allocation” is used in the present disclosure to generally refer to the process by which spectrum is allocated, allotted, and assigned to licensed users.
In view of this increasing demand for spectrum, a dynamic spectrum access (DSA) system may be used to share available spectrum among multiple users. A DSA system, for example, may include a Spectrum Access System (SAS) that manages access to a shared spectrum, such as the 3.5 GHz band recently made available for commercial use in the United States. In another example, a DSA system may be used to share access to unlicensed spectrum, such as TV Whitespace. Coordinating and managing multi-user access to a shared spectrum presents challenges in a DSA system.
Spectrum sensing may be used to enable efficient use of a shared spectrum. A spectrum sensor in a DSA system may monitor a frequency channel in the shared spectrum to determine if that channel is being used by other users. For example, information obtained from a spectrum sensor may enable a user of a DSA system to identify if the frequency channel is not currently being used or is being used by users having higher-priority access permissions, such as military users.
Spectrum sensing typically involves detecting the amount of energy or power (i.e., energy per unit time) received in a frequency channel, for example using a radiometer designed for that frequency. This spectrum sensing technique, however, requires knowledge of the noise and interference characteristics of the frequency channel and typically cannot detect signals having low signal-to-noise ratios (SNR). In the increasingly congested wireless environment, these limitations of spectrum sensors can be prohibitive because the received signals may have low SNR and the noise and interference environment is highly variable.
Air Route Surveillance Radar Model 4 (ARSR-4) and Ship Air Surveillance Radar AN/SPN-43 are examples of radar systems that are currently being used by the U.S. government in the L-band and the 3.5 GHz band, respectively. Other radar systems may operate in frequency bands, for example, adjacent to the 3.5 GHz band. ARSR-4, for example, is a frequency-modulated continuous-wave radar system located on the United States' borders and coastlines as illustrated in FIG. 1. The ARSR-4 radar system transmits signal pulses, where each pulse consists of two complex linear chirps at different center frequencies. The rise and fall times and signal shape of an ARSR-4 pulse may be approximated by a raised cosine function. Each linear chirp within an ARSR-4 pulse is itself a signal having a frequency that increases linearly as a function of time.
Spectrum sensing of incumbent signals, such as ARSR-4 and AN/SPN-43 radar signals and other terrestrial and satellite signals, is of particular interest in DSA systems used to manage spectrum usage in a shared spectrum, since the incumbent signals are typically afforded a higher-priority access to the shared spectrum. Accordingly, the DSA system must be aware of the presence of these higher-priority signals to avoid interference with them. For example, the DSA system may employ a SAS that has the capability of sensing users' spectrum usage. The SAS may ensure protection of incumbent signals by managing the assignment and re-assignment of frequency channels to users under the system's management based on the sensing results.